Many conventional scaffoldings, such as the ones described in the U.S. Pat. No. 4,809,814 delivered in 1989 to Jean ST-GERMAIN (hereafter the '814 patent) and in the U.S. Pat. No. 3,318,414 delivered in 1967 to MEEK (hereater the MEEK patent), comprise one or more towers along which corresponding sleeve members are movable. The sleeve members can either slide along the towers (as in the MEEK patent) or they can be equipped with a plurality of idle wheels rotatably attached thereto and roll along the towers (as in the '814 patent). Each sleeve member supports a work platform allowing the workers an even surface to stand on while they accomplish their tasks. The sleeve members (and consequently the work platforms) can be raised or lowered at a desired height.
To raise or lower a platform along a tower, a few systems have been introduced in the prior art.
A very common system is the use of a rack and gear assembly. The tower is equipped with a vertical elongated rack which has equally spaced teeth that are complementary to those of a gear wheel. The shaft of the gear wheel is installed on the sleeve member supporting the work platform and is linked to power means (either manual, electrical or other). When the power means are activated, the gear wheel will engage the rack to raise (or lower) the platform.
A first problem with this system is that the whole load of the work platform and its contents is supported by a single (or possibly two) tooth of the gear wheel which bears on a corresponding rack tooth at a given time. This is indesirable, since the load bearing on the tooth can be rather significant (1000 pounds or more). It is thus possible that a rack or a gear wheel tooth will break under the load, since the surface of said tooth is relatively small for supporting such a load.
Another problem associated with this system is that the gear wheel shaft itself will occasionally break under the torque induced by the carried load. This is partially due to a too small reducing gear ratio: for example, while a 5:1 ratio may be good under most circumstances, the wear of the system and/or an especially heavy load may induce constraints that are greater than what is allowable in the system, resulting in the breaking of the shaft.
Yet another problem with the above-mentioned system is that the rack and gear assembly requires a good deal of grease for lubrification of the rack and gear wheel connection, because of the high friction resulting therefrom. This grease may be undesirable in certain specific circumstances, especially when a clean or sterile environment is needed.
Another problem with the rack and gear assembly arises when dealing with cement or another similar material. Indeed, during the cementing process, the cement will often embed itself in the grooves between two sucessive rack teeth, to be compacted there by the gear wheel teeth as they lodge themselves in these grooves while the wheel rolls along the rack. The gear wheel will consequently have less or no surface to bear upon since the space between two rack teeth will be at least partially filled with cement. A lifting aparatus thus impaired can no longer be used until it is cleansed of the cement, which can be a strenuous task if the cement is dry.
The prior art shows other systems to raise a work platform, notably the two previously-mentioned patents.
The MEEK patent shows a pulley 37 from which depends the two extremities of a cable 41. A winch and drum mechanism can be activated by means of a winch handle 42 to raise or lower the platform manually by rolling up cable 41, cable 41 being attached to the platform.
The '814 patent shows a platform raising system (FIG. 7) comprising an arm 84 and a ram member 106 that work together to raise (or lower) the platform 42. Ram member 106 will alternately extract and retract its piston rod 112 which will correspondingly raise and lower its hook member 118. The latter will engage successive steps or rungs 38 to raise work platform 42. While hook member 118 is being raised, arm 84 (and safety arm 84') pivotably engages a rung 38 to sustain work platform 42.
Several alternatives have been used in other inventions, but they remain substantially similar to one or the other of the above-mentioned systems.